1. Field of the Invention
The present invention relates to an illumination optical system and a projector, and specifically relates to an illumination optical system for illuminating an image display surface of a display device such as a digital micromirror device or a liquid crystal display (LCD), and a projector provided therewith.
2. Description of Related Art
In recent years, there have been growing needs for projection by using polarization in a DLP projector (DLP (digital light processing): registered trademark of Texas Instruments Limited in USA). One example of such projection is 3D projection which is performed using polarization glasses, and another is high-contrast projection which is performed using a polarization screen. Polarization is used in either projection; however, since DLP projectors typically use randomly polarized light, if projection is performed with a DLP projector by using polarization, a half or more of the amount of the light is inefficiently lost without being used. This makes it difficult to realize projection onto a large screen. Patent Documents 1 to 4 each propose a projector in which an improved method is used to convert polarization or combine light beams from two lamps, but none of them propose a bright illumination optical system that uses a rod integrator suitable for the DLP and that also uses two lamps and converts polarization.    Patent Document 1: JP-A-2000-131647    Patent Document 2: JP-A-2006-30330    Patent Document 3: JP-A-2007-140344    Patent Document 4: U.S. Pat. No. 6,222,674B1
The illumination optical system proposed in Patent Document 1 is configured such that polarization conversion is performed at an entrance face of a rod integrator by displacing imaging positions of light beams on a polarization-by-polarization basis. Such polarization conversion helps make effective use of the amount of light, but, since no mechanism for combining light beams from two-lamps is adopted, a projection image obtained by using this illumination optical system cannot be sufficiently bright. In addition, a prism is used as a polarization beam splitter, and this limits the direction of polarization separation and the polarization component, and thus accordingly limits the layout, which is inconvenient.
The illumination optical system proposed in Patent Document 2 is configured such that light beams from two lamps are combined at an entrance face of a rod integrator by making light beams from the two-lamp sources form images at the same position. However, since no mechanism for polarization conversion is adopted, effective projection cannot be performed by using polarization, which is inconvenient.
The illumination optical system proposed in Patent Document 3 is configured such that light beams from two lamps are combined by making light beams from the two-lamp sources form images at different positions in an entrance face of a rod integrator. However, since no mechanism for polarization conversion is adopted, effective projection cannot be performed by making use of polarization, which is inconvenient.
The illumination optical system proposed in Patent Document 4 is configured such that both the combining of light beams from two lamps and polarization conversion are performed. However, since a lens array is used as an integrator, illumination light beams from lenses of the lens array are liable to be displaced from each other on a display device, where they should be superimposed on each other, and spread light resulting from the displacement becomes ghost light and degrades the quality of projection. In addition, because there is no room for disposing a color wheel, it is not suitable for a single-plate type DLP.